ESCHERICHIA-COLI TOL-PAL MUTANTS FORM OUTER-MEMBRANE VESICLES

Mutations in the tol-pal genes induce pleiotropic effects such as rele ase of periplasmic proteins into the extracellular medium and hypersen sitivity to drugs and detergents. Other outer membrane defective strai ns such as tolC, Ipp, and rfa mutations are also altered in their oute r membrane permeability. In this study, electron microscopy and Wester n blot analyses were used to show that strains with mutations in each of the tol-pal genes formed outer membrane vesicles after growth in st andard liquid or solid media. This phenotype was not observed in tolC and rfaD cells in the same conditions. A tolA deletion in three differ ent Escherichia coli strains was shown to lead to elevated amounts of vesicles. These results, together with plasmid complementation experim ents, indicated that the formation of vesicles resulted from the defec t of any of the Tol-Pal proteins. The vesicles contained outer membran e trimeric porins correctly exposed at the cell surface. Pal outer mem brane lipoprotein was also immunodetected in the vesicle fraction of t ol strains. The results are discussed in vie cv of the role of the Tol -Pal transenvelope proteins in maintaining outer membrane integrity by contributing to target or integrate newly synthesized components of t his structure.